1. Field of the Invention
This invention relates to low thermal expansion ceramic composite compositions based upon zirconium titanate and Al.sub.2 TiO.sub.5, and their solid solutions, which have high melting points and excellent phase stability at high temperatures or cycling conditions.
2. Description of Previously Published Art
Materials with excellent thermal shock resistance are required for a wide range of service applications including automotive catalyst supports, molten metal filters, welding fixtures, kilnware, etc. For high temperatures above 1000.degree. C., this necessitates the use of ceramic materials. It is known that the thermal shock resistance depends upon the characteristics of the material, including strength, Young's Modulus, coefficient of thermal expansion, thermal conductivity, and the physical configurations of the shape. The coefficient of thermal expansion is especially critical since low values mean the material undergoes minimal dimensional changes over a wide temperature range.
Consequently, the use of low expansion materials in applications which see rapid, gross temperature changes, usually designated delta T by those skilled in the art, is highly desirable. These materials are not prone to the large stress buildups and consequent fracturing with extreme thermal cycling or large delta T values.
A wide variety of crystalline ceramics have been utilized for their low thermal expansions/high melting points for such applications. Crystalline materials may have isotropic or anisotropic thermal expansions; that is, expansions may be the same, similar, or very different for each of the crystallographic directions. Lithium aluminum silicate (Beta-spodumene) shows marked anisotropy such that its thermal expansion alpha.sub.24.degree.-1000.degree. C. along its c-axis is -17.6.times.10.sup.-6 .degree.C..sup.-1 while the two values normal to the c-axis are each +8.2.times.10.sup.-6 .degree.C..sup.-1. Its melting point limits the service use to about 1200.degree. C. Cordierite, Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18, is widely used to temperatures of about 1450.degree. C. Cordierite has crystallographic expansions of -1.1.times.10.sup.-6 .degree.C..sup.-1 along its c-axis and +2.90.times.10.sup.-6 .degree.C..sup.-1 normal to the c-axis. However, the average expansions of a polycrystalline cordierite ceramics range between 0.7 and 1.5.times.10.sup.-6 .degree.C..sup.-1.
Aluminum titanate, Al.sub.2 TiO.sub.5, has expansion values of -3.0, +11.8 and +21.8.times.10.sup.-6 .degree.C..sup.-1 for its three crystallographic directions. Aluminum titanate mixed with mullite to form a mullite-Al.sub.2 TiO.sub.5 (MAT) particle composite has experienced much research for thermal shock use. Its polycrystalline expansion ranges between approximately 0.5 and 1.5.times.10.sup.-6 .degree.C..sup.-1. The resultant material properties reflect a compromise of the low strength/low expansion of Al.sub.2 TiO.sub.5 and high strength/moderate expansion of mullite. These materials are highly complex due to the propensity of the Al.sub.2 TiO.sub.5 to severely microcrack, and the decomposition of Al.sub.2 TiO.sub.5 to Al.sub.2 O.sub.3 and TiO.sub.2 between 800.degree.-1250.degree. C., above which they recombine to again form Al.sub.2 TiO.sub.5. Much work has been done to control the decomposition of Al.sub.2 TiO.sub.5 by the addition of small amounts of stabilizers, including MgO and Fe.sub.2 O.sub.3, into solid solution. This is recognized in the prior art, notably U.S. Pat. No. 2,776,896.
Other references note some beneficial results from the addition of rare earths, SiO.sub.2, CaO, BaO, etc. to Al.sub.2 TiO.sub.5 but these have received less attention. However, the complex interrelationships between microcracking, phase stability and physical/mechanical properties make Al.sub.2 TiO.sub.5 or mullite-Al.sub.2 TiO.sub.5 a very difficult material to control.
Zirconia has received much attention in the ceramics industry and there are many commercial ZrO.sub.2 products and processes. The ability of ZrO.sub.2 to be stabilized with additives such as Y.sub.2 O.sub.3, CaO or MgO to maintain a metastable high temperature phase has been recognized. By adding unstabilized or partially stabilized ZrO.sub.2 to a constraining matrix of a second ceramic such as Al.sub.2 O.sub.3 or mullite, the ZrO.sub.2 imparts a greater toughness to the resultant ceramic composite, thereby enhancing its mechanical properties. The affect of the ZrO.sub.2 added is strongly dependent upon the amount of ZrO.sub.2, as well as the particle size (generally on the order of 0.5 microns), which are essential to achieve effective toughening. See U.S. Pat. No. 4,316,964.
Al.sub.2 TiO.sub.5 has been combined with various materials. For example, Japanese Patent Publications Nos. 55062840, 55062841, 55062842 and 55062843 disclose adding zircon (ZrSiO.sub.4) and alkaline earth metal oxides or hydroxides. Japanese Patent Publication No. 55063387 discloses zirconium (Zr) and alkaline earth metal oxide. German Pat. No. 19 15 787 discloses adding zirconium silicate. Japanese Patent Publication No. 52023113 discloses zirconia or silica. Other references disclosing zirconia are L. M. Silich et al in Steklo, Sitally i Silikaty, vol. 13, pages 110-14 (1984), Soviet Pat. No. 899600 and Japanese Patent Publication No. 55121967. Japanese Patent Publication No. 79025045 discloses adding to aluminum titanate, zirconia and Y.sub.2 O.sub.3, CeO.sub.2 and/or La.sub.2 O.sub.3. Japanese Patent Publication No. 60046970 discloses adding 1-10 wt % of at least one of Fe.sub.2 O.sub.3, SiO.sub.2, MgO, ZrO.sub.2, cordierite, mullite and clay to a composition of 100 parts by weight of aluminum titanate, 0.5-10 parts by weight of Li.sub.2 O and 4.5-30 parts by weight of SiO.sub.2. Belgian Patent No. 898,604 discloses zircon and alumina which can have further added titanium oxide.
ZrTiO.sub.4 and its solid solutions (especially with SnO.sub.2) have been extensively utilized in the electronics industry due to their good dielectric properties in microwave regimes. ZrTiO.sub.4 has also been used as a base for multi-phase pigments for high temperature applications. However, there is little evidence in the literature indicating that ZrTiO.sub.4 has been considered for technical ceramics applications such as those proposed herein. A recent study by McHale and Roth (1986) describes the complex series of continuous phase transformations which zirconium titanate undergoes below approximately 1150.degree. C. It was found that when the stoichiometry 1 ZrO.sub.2 :1 TiO.sub.2 is reacted, ZrTiO.sub.4 does not form as previously believed. Instead, some ZrO.sub.2 is precipitated out and a continuous series of possible zirconium titanate compositions may form. The propensity of zirconium titanate to form solid solutions, and the important role of minor impurities on the behavior of zirconium titanate compositions is noted.
The interrelationship between the microstructure of a ceramic and the compositional effects of the constituents is known in the literature. By microstructure, the grain sizes, grain orientations, porosity, distributions of phases and other physical characteristics of the ceramic are taken into account. Mullite-Aluminum Titanate composites are used by way of illustration. It is known that Aluminum Titanate experiences microcracking due to its high degree of crystal anisotropy which, in turn, leads to a macroscopic low thermal expansion. However, the size of the microcracks is directly related to the size of the aluminum titanate grains in the microstructure. Hence, thermal expansion reflects the size of available microcracks which, in turn, is dependent upon grain size. Too small a grain size does not allow effective microcracking and there will be no effective reduction the thermal expansion. Conversely, too large of a grain size may create very large microcracks which gives a low thermal expansion but can also lead to a significant reduction of the mechanical integrity of the ceramic. Thus, a mullite-aluminum titanate ceramic with a 15 micron aluminum titanate average grain size gives alpha.sub.24.degree.-1000.degree. C. =1.5.times.10.sup.-6 .degree.C..sup.-1, while an identical composition with a finer, 1-2 micron aluminum titanate average grain size gives alpha.sub.24.degree.-1000.degree. C. =4.5.times.10.sup.-6 .degree.C..sup.-1, and an intermediate grain size of 10 microns gives alpha.sub.24.degree.-1000 .degree. C.=2.8.times.10.sup.-6 .degree.C..sup.-1. This effect may be further altered by the addition of small amounts of phase stabilizers to aluminum titanate. Since the stabilizers reduce crystal anisotropy to inhibit decomposition, it follows that such solid solutions may also change microcrack size and consequently, thermal expansion. Therefore, by manipulating grain size versus composition of the aluminum titanate, a balance can be achieved between chemical composition of the constituents and microstructural relationships to create changes in properties, for example, thermal expansion.